1. Field of Technology
The present invention relates to a thermal printer for printing to a print medium at a controlled print medium speed relative to a print head, a thermal printer control method, and a printing system.
2. Description of Related Art
Thermal printers hold a print medium such as thermal paper between the thermal print head and a platen roller and advance the paper by rotating the platen roller. The thermal print head has heating elements (dots) arrayed in a line (one dot line) across the width of the paper, and applies current to selected dots in this dot line to produce heat and cause the thermal paper to change color. The thermal printer prints by energizing the thermal print head while advancing the thermal paper. Torque for rotating the platen roller is transferred from a rotational drive source such as a stepping motor through a transfer mechanism (a gear train) to the platen roller.
The printing speed of a thermal printer is determined by various parameters, including the energizing voltage applied to the thermal print head, the print duty (the ratio of printed dots to the number of total dots in one dot line), the temperature, printing pattern, print data communication speed, and the amount of time required for internal data processing. These parameters are hereinafter referred to as the “print speed control factors”. A change in one or more of these parameters changes the print head energizing time and print speed. The print head energizing time and print speed are adjusted according to change in these print speed control factors in order to achieve the best print quality. See, for example, Japanese Unexamined Patent Appl. Pub. H06-55750. The print speed of a thermal printer is equal to the paper feed rate because printing occurs while the paper is advanced.
The change in print speed while printing with a conventional thermal printer is shown in FIG. 8.
FIG. 8 shows an example in which the print speed changes greatly in period a (decelerating in curve (A) and accelerating in curve (B)), and then frequently changes slightly in period b according to the change in the print speed control factors (including print duty). When the print speed frequently changes slightly in this way, the mechanical rigidity of the transfer mechanism and backlash in the gear train, including deformation of the rubber platen roller and the inertia of the motor, gears, and other rotating parts, affect print quality. More specifically, these factors produce an offset between the timing of the signal (such as the stepping motor excitation signal) causing the rotational drive source to turn and the timing of actual platen roller rotation (the rotational position of the platen roller). The timing of the signal (strobe signal) for energizing (heating) the thermal print head is normally determined based on the timing of the signal causing the rotational drive source to rotate.
Therefore, if the timing of actual platen roller rotation is offset from the signal causing the rotational drive source to rotate, the timing of platen roller rotation is also offset from the timing at which the thermal print head energizes and heats (the timing at which the printed dots are formed). This causes the distance between printed dots in the paper transportation direction to vary, resulting in an inconsistent printing pitch and a loss of print quality.
There is a particular tendency for a pronounced deviation in printing pitch when the print speed frequently changes slightly after a significant change in print speed because the timing of actual platen roller rotation is not stable.
This is further described below using receipt printing by a thermal printer in a POS terminal by way of example. The store name and logo, and purchase information including the name and price of each purchased product, are typically printed on a receipt. The store name and logo are generally printed first in the header at the beginning of the receipt, and the purchase information is then printed in text following the header. The print duty differs greatly during logo printing for printing graphic data and when printing text. More particularly, the print duty is high during logo printing and low when printing text. The print speed control factors, including the print duty, energizing voltage, and thermal print head temperature, therefore change greatly when changing from logo printing (period a in FIG. 8(A)) to text printing (period b in FIG. 8(A)), and the print speed therefore also changes greatly. During text printing the print duty tends to frequently change slightly from dot line to dot line. As a result, if purchase information or other text is printed in period b after logo printing ends, there are also frequent slight changes in the print speed control factors and the print speed frequently changes slightly. The dot pitch between the printed characters therefore varies in period b, and print quality drops.
The thermal printer, the control method, and the printing system of the present invention prevents variation in the dot pitch in the printed output of the thermal printer as a result of the print speed frequently changing slightly after a great change in the print speed.